This invention relates to aqueous wellbore service fluids, including drilling fluids, completion fluids, work over fluids, packer fluids, fracturing fluids and the like, which may be employed in various well servicing operations. More specifically, it relates to thickened, substantially solids-free high electrolyte-containing aqueous fluids which are employed as a base fluid to prepare many types of wellbore service fluids.
Essentially solids-free aqueous fluids containing electrolytes have some advantages over clay-based fluids for preparing wellbore service fluids because: (a) they do not normally contain undesirable solids which can cause formation damage, (b) they contain hydration inhibiting materials such as potassium chloride, calcium chloride or the like, which are important to prevent damage to clay containing formations, and (c) they can be prepared over a wide range of densities.
The viscosity of high electrolyte-containing aqueous fluids is, however, difficult to control because of the high electrolyte concentration. Thickened fluids are desirable for carrying solids, e.g., in cleaning out wells, drilling and the like. Likewise, thickened fluids resist water loss, which may be damaging to petroleum producing subterranean formations.
Hydroxy alkyl celluloses have been employed to thicken electrolyte-containing aqueous fluids to improve the solid carrying capacity thereof. Likewise, starch has been employed to aid in water loss control of these fluids, but with limited success. However, these materials are difficult to disperse and dissolve in concentrated electrolytes at ambient temperature; the viscosity of the resulting solutions tend to decrease with an increase in temperature; and the hydroxy alkyl celluloses are subject to shear degradation under normal operating conditions.
Certain quaternary ammonium salts have been shown to impart viscoelastic properties to aqueous solutions, S. Gravsholt "Viscoelasticity in Highly Dilute Aqueous Solutions of Pure Cationic Detergents", Journal of Colloid and Interface Science, Vol. 57, No. 3, December 1976, pp. 575-577. Gravsholt showed that cetyl trimethyl ammonium bromide would not impart viscoelastic properties to water but that cetyl trimethyl ammonium salicylate and certain other aromatic anion-containing quaternary amines would. In U.S. Pat. No. 3,292,698, a mixture of cyclohexyl ammonium chloride and undecane-3-sodium sulfate was taught to induce viscoelastic properties to a formation flooding liquid containing less than about 3.5 percent by weight of sodium chloride. Higher levels of sodium chloride were said to destroy the viscoelastic properties of the fluid. In British Pat. No. 1,443,244, a specific ethoxylated or propoxylated tertiary amine is employed to thicken an aqueous solution of a strong mineral acid. U.S. Pat. No. 3,917,536 teaches that certain primary amines may be employed in subterranean formation acidizing solutions to retard the reaction of the acid on the formation. The amine may be more readily dispersed into the acid solution with the use of a dispersing agent such as a quaternary amine.
It is a feature of the present invention to provide a high electrolyte-containing aqueous wellbore service fluid which has improved viscosity characteristics over a wide range of wellbore conditions; is easier to prepare at the well site and has better shear stability and consistent viscosity over a wide temperature range.
The improved aqueous wellbore service fluid of the present invention can be employed in well-known wellbore services such as, perforation, clean-up, long term shut-ins, drilling, placement of gravel packs, and the like. These services are well known in the art and are taught, for example, in U.S. Pat. Nos. 3,993,570; 3,126,950; 2,898,294 and C. M. Hudgens et al "High Density Packer Fluids Pay Off in South Louisiana": World Oil, 1961, pp. 113-119. The teachings of these publications are specifically incorporated herein by reference.
As employed herein, "ppg" means pounds per gallon. Also, when "percent" or "%" are employed, they mean percent by weight unless otherwise specified.